Atomic structure

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					         HISTORY OF THE ATOM

460 BC   Democritus develops the idea of atoms



              he pounded up materials in his pestle and

              mortar until he had reduced them to smaller

              and smaller particles which he called



                      ATOMA
                  (greek for indivisible)
       HISTORY OF THE ATOM

1808   John Dalton



            suggested that all matter was made up of

            tiny spheres that were able to bounce around

            with perfect elasticity and called them



                     ATOMS
       HISTORY OF THE ATOM

1898   Joseph John Thompson



           found that atoms could sometimes eject a far

           smaller negative particle which he called an




                 ELECTRON
         HISTORY OF THE ATOM
1904
Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere surrounded

by a soup of positive charge to balance the electron's charge

                             like plums surrounded by pudding.



                                 PLUM PUDDING
                                       MODEL
       HISTORY OF THE ATOM

1910   Ernest Rutherford


            oversaw Geiger and Marsden carrying out his
            famous experiment.

            they fired Helium nuclei at a piece of gold foil
            which was only a few atoms thick.

            they found that although most of them
            passed through. About 1 in 10,000 hit
         HISTORY OF THE ATOM

                                                 gold foil
                      helium nuclei




                                                helium nuclei


They found that while most of the helium nuclei passed
through the foil, a small number were deflected and, to their
surprise, some helium nuclei bounced straight back.
         HISTORY OF THE ATOM


Rutherford’s new evidence allowed him to propose a more
detailed model with a central nucleus.


He suggested that the positive charge was all in a central
nucleus. With this holding the electrons in place by electrical
attraction



However, this was not the end of the story.
       HISTORY OF THE ATOM

1913   Niels Bohr


              studied under Rutherford at the Victoria
              University in Manchester.

              Bohr refined Rutherford's idea by adding
              that the electrons were in orbits. Rather
              like planets orbiting the sun. With each
              orbit only able to contain a set number of
              electrons.
Bohr’s Atom

                        electrons in orbits




              nucleus
            HELIUM ATOM
                                             Shell
proton




                      +
                          N
                                      -
                          +
           -          N




electron                                  neutron

What do these particles consist of?
    ATOMIC STRUCTURE


Particle     Charge      Mass

proton     + ve charge    1

neutron    No charge      1

electron   -ve charge     nil
     ATOMIC STRUCTURE


            2

He
                Atomic number
                 the number of protons in an atom




            4    Atomic mass
                  the number of protons and
                  neutrons in an atom


number of electrons = number of protons
             ATOMIC STRUCTURE

    Electrons are arranged in Energy Levels or

    Shells around the nucleus of an atom.


•     first shell         a maximum of 2 electrons

•     second shell        a maximum of 8 electrons


•     third shell         a maximum of 8 electrons
            ATOMIC STRUCTURE

There are two ways to represent the atomic

structure of an element or compound;


       1.     Electronic Configuration



         2.    Dot & Cross Diagrams
     ELECTRONIC CONFIGURATION

With electronic configuration elements are represented

numerically by the number of electrons in their shells

and number of shells. For example;


 Nitrogen                  configuration = 2 , 5

                                                       7


                                                   N
  2 in 1st shell
                       2    +   5 = 7
  5 in   2nd   shell
                                                       14
      ELECTRONIC CONFIGURATION

Write the electronic configuration for the following
elements;

          20                 11              8
a)   Ca           b)   Na           c)   O
                             23              16
          40

  2,8,8,2              2,8,1             2,6

          17                14               5
d)   Cl           e)   Si           f)   B   11
          35                28

     2,8,7             2,8,4             2,3
        DOT & CROSS DIAGRAMS

With Dot & Cross diagrams elements and compounds

are represented by Dots or Crosses to show electrons,

and circles to show the shells. For example;
                              X




                                               N
Nitrogen             X X      N     X X            7



                                                   14
                              XX
        DOT & CROSS DIAGRAMS

Draw the Dot & Cross diagrams for the following
elements;
           8                17
                                       X
    a) O            b)  Cl 35 X                X
          16                           X
        X                        X           X
              X
            X              X X X Cl X X            X
  X         O   X
                                 X           X
            X                          X         X
      X       X                X
                                      X
                     SUMMARY
1. The Atomic Number of an atom = number of
     protons in the nucleus.

2. The Atomic Mass of an atom = number of
     Protons + Neutrons in the nucleus.

3.   The number of Protons = Number of Electrons.

4.   Electrons orbit the nucleus in shells.


5.   Each shell can only carry a set number of electrons.

				
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